Polyolefins such as polyethylene and polypropylene may be prepared by a gas phase or slurry polymerization process, which involves the polymerization of olefin monomer with the aid of catalyst and optionally, if required depending on the used catalyst, a co-catalyst. Following such polymerization process, a polymerization effluent is produced comprising polymer solids in a fluid that contains unreacted monomer, and optionally unreacted comonomer.
In gas phase polymerization, polyolefin processes are frequently involved with the separation between monomers such as ethylene or propylene and inert gas like nitrogen, hydrogen . . . . Ethylene is ideally recycled to the reactor while other components have to be purged in order to avoid accumulation. An effective recycling of ethylene needs a separation between monomer (namely ethylene or propylene) and inert components.
In slurry polymerization, polyolefin processes are frequently involved with the separation between diluent such as isobutane, and monomers such as ethylene and inert gas like nitrogen, hydrogen . . . .
The composition of the stream to be separated depends on the process, but gas phase as well as slurry process have, at one point of the process, the necessity to split these components in order to be able to recycle ethylene.
It is highly desirable to recover the unreacted monomer, and unreacted comonomer, since there is an economic interest in re-using these separated components in a polymerization process.
Adsorption processes have been proposed to separate hydrocarbons from purge or vent gas streams produced by processes similar to polyethylene and polypropylene reactor systems.
EP0722953 discloses recovery of hydrocarbons from product purge gas generated in the synthesis of a polyalkene product, comprises contacting the product purge gas comprising inert gas, unreacted alkene monomer, one or more heavy hydrocarbons, and one or more intermediate hydrocarbons, in a scrubber with a scrub liquid feed comprising at least one of the one or more intermediate hydrocarbons, thereby absorbing at least a portion of the one or more heavy hydrocarbons from the product purge gas into the scrub liquid. A vapor overhead comprising inert gas and at least a portion of the alkene monomer and an absorber bottoms scrub liquid comprising at least a portion of the one or more heavy hydrocarbons are removed from the absorption zone. The process disclosed in this document is cumbersome as it involves several counter-flow heat exchangers for recovering the alkene monomer from the vapor overhead.
It is therefore an aim of the present invention to provide a separation process between ethylene and inert gas, as simple as possible, and with little demand in energy.